Radio frequency (RF) laterally diffused metal oxide semiconductor (LDMOS) devices are RF power devices that have been widely used in radio and television base stations, mobile communications base stations, radars and many other applications. They have a variety of advantages such as high linearity, high gain, high withstand voltage and great output power. When sorted by working voltage, RF LDMOS devices can be categorized into 28 V and 50 V ones with a required breakdown voltage of 70 V and 120 V, respectively. FIG. 1 shows a common prior art N-type RF LDMOS device including a P-type substrate 1 and a P-type epitaxial layer 2 formed on the P-type substrate 1. A P-type channel region 5 and a lightly-doped N-type drift region 6 are both formed in P-type epitaxial layer 2 and make contact with each other laterally. The RF LDMOS device also includes a heavily-doped N-type drain region 7 in the lightly-doped N-type drift region 6, a source region 8 in the p-type channel region 5, and a P-type sinker 10 in the P-type epitaxial layer 2. The P-type sinker 10 extends downward to the top surface of the P-type substrate 1 and contacts with both P-type channel region 5 and the source region 8. Portions of each of the heavily-dope N-type drain region 7, the source region 8 and the P-type sinker 10 are covered by a metal silicide layer 9. A gate oxide layer 3 is formed on a top of the P-type epitaxial layer 2. A gate metal silicide layer 9 and a polysilicon gate 4 are stacked on the gate oxide layer 3 in this order from the top downwards. A Faraday shield 11 formed of a metal layer covers a portion of the polysilicon gate 4 and a portion of the gate oxide layer 3 proximal to the heavily-dope N-type drain region 7. In this design, a length of the lightly-doped N-type drift region 6 (specifically, a distance between facing sides of heavily-dope N-type drain region 7 and polysilicon gate 4) and the Faraday shield 11 that acts as a field plate for electric field distribution regulation together determine whether the RF LDMOS device can have a high withstand voltage. On the other hand, the device also forms an equivalent parasitic NPN transistor with the heavily-dope N-type drain region 7 and the N-type drift region 6 jointly serving as a collector, the P-type channel 5 and the P-type sinker 10 together serving as a base, and the source region 8 serving as an emitter. When in use, the emitter and the base of this parasitic NPN transistor are interconnected and grounded, which causes the P-type channel region 5 to be grounded via the P-type sinker 10 and thereby creates an equivalent base resistance RB. Meanwhile, as shown in FIG. 2, which is an equivalent circuit diagram of the RF LDMOS device, a reverse-biased parasitic diode is formed between the N-type drift region 6 and the P-type channel region 5. During a normal operation of the device, the heavily-dope N-type drain region 7 may be applied with a working voltage and an RF signal, which sum to a value that is nearly equal to the breakdown voltage of the RF LDMOS device, or occasionally with a pulse voltage with the peak value that is greater than the breakdown voltage. This requires both of a reverse breakdown voltage of the equivalent parasitic diode and a snapback voltage of the equivalent parasitic transistor to be about 20 V higher than the breakdown voltage of the RF LDMOS device. To meet this requirement, in addition to a reverse breakdown voltage about 20 V higher than the breakdown voltage, the diode should also have a low leakage current and a low equivalent base resistance RB. FIG. 3 is a diagram depicting characteristic curves of drain voltage versus drain current of the common RF LDMOS devices which have working voltages of 28V and 50V respectively. As seen in FIG. 3, snapback occurs at about 90 V in the RF LDMOS device with a working voltage of 28V and between 140 V and 150 V in the RF LDMOS device with a working voltage of 50V. For an RF LDMOS device, a higher snapback voltage means a better performance.
Different from the above described common RF LDMOS devices that utilize the P-type sinker 10 formed by long-time diffusion as an electric sinker, which forms a lower base resistance RB with the P-type channel region 5, there is another type of RF LDMOS device, as shown in FIG. 4, which uses a tungsten plug as an electric or heat sinker. Such RF LDMOS device differs in structure from that shown in FIG. 1 in including a tungsten plug 13 instead of the P-type sinker 10 and additionally including a P-type channel connecting region 14. However, although the metal tungsten plug is capable of reducing the electrical resistance with the substrate and facilitating heat dissipation, as this RF LDMOS device still keeps a relative high base resistance RB, it is still possible for snapback to occur which may lead to burnout or other withstand voltage failure of the device.